Covalent hydrates as intermediates in heterocyclic rearrangements. Part 3. The alkali-catalysed transformations of 1,5-diamino-1H-s-triazolo[1,5-c]quinazolinium bromide

Abstract

The action of alkali on 1,5-diamino-1H-s-triazolo[1,5-c]quinazolinium bromide leads to parallel reactions that obey the same unusual rate law: a rate that varies with the square of [OH^–] near pH 10, attenuating to the first power at high pH. The first reaction involves the generation of an anion believed to be the cyanamide (6) whose pK_a value lies below that of the original base, so that the neutral species is always metastable; a situation believed to be unique. This process is reversible. The second is the generation, via a covalently hydrated intermediate, of a basic species believed to be the amidine (9) which on acidification cyclises to the triazoloquinazolone (10) with loss of hydrazine. An unequivocal synthesis of this triazoloquinazolone is described. The rationale of these reactions is discussed in relation to factors known to influence the stability of covalent hydrates and the facility of cyclisation processes.